From the teaching described in the patents U.S. Pat. Nos. 3,703,843 and 3,861,269, a fastening screw is known for attaching together parts consisting of different materials with different strengths.
The fastening screw comprises a drive head from which extends a cylindrical body provided with two cylindrical threads of identical pitch. Over ⅓ of its length, the cylindrical body comprises two cylindrical threads arranged either at its pointed free end (U.S. Pat. No. 3,703,843) or immediately below the drive head (U.S. Pat. No. 3,861,269).
From the teaching described in the patent EP 1991145, a bone screw is also known which includes a threaded body with cylindro-conical profile, provided at one of its ends with a drive head or with a connector for attaching a linking rod of an osteosynthesis device.
The cylindro-conical body comprises, below its drive head or below the connector, a first threaded section including two helical threads of identical pitch extended by a second threaded section consisting of a single helical thread originating from the first threaded section.
In the case of a bone screw, one notes that the bone tissue into which said screw is implanted exhibits different strengths between its central portion consisting of spongy bone and its peripheral portion consisting of cortical bone.
Indeed, the bone tissue consists of a hard and dense component, the cortical bone, covering a trabecular component, and the less dense and less hard spongy bone. These two components form a continuum with a density gradient that increases as it comes closer to the cortical bone.